1. Field of the Invention
The present invention relates to an ink-jet printing method which is capable of reproducing a designed color of ink on a printing material and permits the provision of images high in quality, and a method of producing a print.
2. Related Background Art
An ink-jet printing method is a recording method in which recording is conducted by generating and flying droplets of an ink by one of various ink ejection systems, for example, an electrostatic attraction system, a system using a piezoelectric element to give an ink mechanical vibration or change, or a system in which an ink is heated to form bubbles in the ink, thereby using the pressure thus produced, and applying the droplets in whole or in part to a printing material such as paper or a plastic film coated with an ink-receiving layer. The ink-jet printing method attracts attention as a printing method Which scarcely produces noise and can conduct high-speed printing and color printing.
As inks for ink-jet printing, inks comprising water as a principal component are mainly used from the viewpoint of safety, recordability, etc. Polyhydric alcohols and/or the like are often added to such inks with a view toward preventing clogging of orifice and improving ejection stability.
Besides, water-soluble dyes represented by direct dyes, acid dyes, basic dyes, reactive dyes and food colors are used as dyes for the inks to be used. Further, dyes of the type that a carboxyl group is contained in their molecules have recently come to be used for the purpose of improving the water fastness of an image formed on a printing medium.
As recording materials suitable for use in ink-jet printing, there have hitherto been used printing paper as described in Japanese Patent Publication No. 3-26665, which comprises a base paper web and a coating layer provided thereon containing finely powdered silica and a water-soluble binder such as polyvinyl alcohol, glossy paper as described in Japanese Patent Publication No. 3-25352, which comprises a cast-coated paper web and a film formed thereon containing polyvinyl alcohol having a saponification degree of from 50 to 90 mole % and a crosslinking agent, and a recording sheet for an over-head projector (OHP) as described in Japanese Patent Application Laid-Open No. 60-220750, which comprises a polyester film and a hydrophilic film provided thereon composed of water-soluble polyvinyl alcohol having a saponification degree of from 70 to 90 mole %.
With the recent improvement in performance of ink-jet printing apparatus, such as speeding up of printing and multi-coloring of images, printing materials for ink-jet are also required to have higher and wider properties. More specifically, they are required to simultaneously satisfy, for example, the following properties:
(1) causing no color deviation from an inherent color of a dye and being able to achieve proper color mixing; PA0 (2) having high ink absorptivity (absorbing capacity being great, and absorbing time being short); PA0 (3) providing dots high in optical density and clear in periphery; PA0 (4) providing dots having a substantially round shape and a smooth periphery; PA0 (5) undergoing scarce changes in the properties even at varied temperatures and humidities and no curling; PA0 (6) undergoing no blocking; PA0 (7) being able to stably store images thereon for a long period of time without deteriorating them (in particular, in a high-temperature and high-humidity environment); and PA0 (8) being stable without undergoing deterioration even when stored for a long period of time (in particular, in a high-temperature and high-humidity environment).
Besides, printing sheets for OHP, and the like are further required to have excellent transparency in addition to the above requirements.
As described above, the dyes of the type that a carboxyl group is contained in their molecules have recently come to be used as dyes for inks with a view toward improving the water fastness of an image formed on a printing material.
An ink prepared by using a dye of this type has an absorbance curve according to variation in pH as illustrated in FIG. 1. More specifically, the ink has a feature that when the pH of the ink is gradually lowered from a pH in a stable region of the ink, its absorbance is rapidly reduced. This is believed to be attributed to the rapid aggregation of the dye at a certain pH. Incidentally, the pH in a stable region of the ink is a pH at which the dye is fully dissolved in the ink and hence undergoes neither aggregation nor precipitation. Therefore, the aggregation of the dye may occur very quickly according to the pH (pH.sub.B) of the surface of a printing material, which may offer such problems that a color deviation from an inherent color of the dye is caused, and proper color mixing cannot be achieved. In particular, printing materials such as a recording sheet for OHP, of which transparency is required, involve a serious problem attributable to strong aggregation of the dye therein that the printed area of such a printing material cannot transmit light due to haze development on its surface, in addition to the problems that a color deviation from an inherent color of the dye is caused, and proper color mixing cannot be achieved.
For example, Japanese Patent Application Laid-Open No. 56-89594 describes a method of suppressing a color deviation by adjusting a difference in pH between an ink and the surface of a printing sheet within 2. However, even the use of such a method cannot solve the problems that a color deviation from an inherent color of the dye is caused, and proper color mixing cannot be achieved if an ink-jet printing ink containing the dye capable of improving the water fastness of images formed is used. More specifically, the dye capable of improving the water fastness rapidly changes the absorbance of an ink containing it depending upon pH. Therefore, reference must be made to the dependence of absorbance of ink on pH.
With the progress of speeding up of printing, increasing of image density, coloring of images and imparting of good water fastness to images, as described above, reduction in color tone and image quality has become a serious problem.